Lead zirconate titanate, which is a typical piezoelectric material containing lead, is used in a variety of piezoelectric devices such as actuators, oscillators, sensors, and filters. However, it has been pointed out that lead components in discarded piezoelectric devices will be dissolved into the soil if the piezoelectric devices are exposed to acid rain and will cause ecological damages. Accordingly, researches and developments of lead-free piezoelectric materials have been actively conducted for providing lead-free piezoelectric devices.
Typical lead-free piezoelectric materials that are currently widely being studied are piezoelectric materials including potassium niobate. However, since the raw material (e.g., potassium carbonate) powder used in synthesis of piezoelectric materials containing potassium has high moisture absorbing properties, it is difficult to exactly weigh a raw material powder at an intended molar ratio. In addition, piezoelectric materials containing potassium niobate (KNbO3) have deliquescency, and the piezoelectricity of some piezoelectric ceramics containing potassium niobate deteriorates with time. Furthermore, in some piezoelectric materials containing potassium niobate, the successive phase transition temperature between tetragonal and orthorhombic crystals is within the operating temperature range (e.g., from 0° C. to 80° C.) of the piezoelectric device. The piezoelectricity significantly varies in a temperature range around the successive phase transition temperature, which causes a problem that the performance of piezoelectric device significantly varies depending on the operating temperature.
NPL 1 also reports on a piezoelectric material not containing lead and potassium, i.e., a solid solution of sodium niobate (NaNbO3) and barium titanate (BaTiO3) (hereinafter, referred to as NN-BT), which is an antiferroelectric material. It is disclosed that the piezoelectric ceramic containing sodium niobate and barium titanate at a ratio of 9:1 had a piezoelectric constant d33 of 147 pC/N.
PTL 1 discloses a niobate-based piezoelectric ceramic having a high electromechanical coupling factor and high heat resistance by adding cobalt oxide (CoO) to NN-BT. However, the insulation is low, i.e., 106Ω or less, and, therefore, polarization of some samples are difficult.
PTL 2 provides a method of producing a niobate-based piezoelectric ceramic having a high Curie temperature and satisfactory piezoelectric properties. It is disclosed that the piezoelectric ceramic of a solid solution of NN-BT and strontium titanate (SrTiO3) had a piezoelectric constant d33 of 14 to 126 pm/V.